Split migrations folder (#21549)

There are too many files in `models/migrations` folder so that I split
them into sub folders.

models/migrations/v1_15/v179.go

@@ -0,0 +1,29 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package v1_15 //nolint
+
+import (
+	"code.gitea.io/gitea/models/migrations/base"
+
+	"xorm.io/xorm"
+	"xorm.io/xorm/schemas"
+)
+
+func ConvertAvatarURLToText(x *xorm.Engine) error {
+	dbType := x.Dialect().URI().DBType
+	if dbType == schemas.SQLITE { // For SQLITE, varchar or char will always be represented as TEXT
+		return nil
+	}
+
+	// Some oauth2 providers may give very long avatar urls (i.e. Google)
+	return base.ModifyColumn(x, "external_login_user", &schemas.Column{
+		Name: "avatar_url",
+		SQLType: schemas.SQLType{
+			Name: schemas.Text,
+		},
+		Nullable:       true,
+		DefaultIsEmpty: true,
+	})
+}